Direct carbon-13 nuclear magnetic resonance (nmr) provides a convenient and facile method for studying the biosynthetic origin of complex natural products. Extension of methods previously developed will provide insight into the nature of the intermediates involved in the formation of complex natural products. NMR of metabolites derived from easily available C13-enriched substrates, notably 1,2-C13 sodium acetate coupled with the useful techniques of continuous wave and proton noise decoupling and mass spectrometry allows quantitative and qualitative determination of isotope enrichment at specific carbon atoms without recourse to extensive chemical degradation. Among the natural products whose biogenetic origin will be established are the lankacidin antibiotics, and the mycotoxins aspyrone and the rubratoxins. These spectrometric methods for biosynthesis study will also be applied to naturally occurring materials of botanical as well as microbial origin. Bibliographic references: H. Seto, L. W. Cary, and M. Tanabe. Utilization of C13-C13 coupling in structural and biosynthetic studies. V. The C13 FT NMR spectrum of sterigmatocystin. Tetrahedron Letters 4491 (1974); H. Seto, L. W. Cary, and M. Tanabe. Utilization of C13-C13 coupling in structural and biosynthetic studies. IV. Penicillic acid. J. Antibiotics 27, 558 (1974).